The present invention relates to an image processing apparatus and method, and a storage medium.
Conventionally, the studies for multiplexing, onto image information, another information that pertains to an image have been extensively made.
In recent years, a technique called digital watermarking, which multiplexes, onto image information such as a photo, painting, or the like, additional information including the copyright holder name, permission/denial of use, and the like so as to be hard to visually detect, and distributes such information via a network such as the Internet or the like, has prevailed.
As another application field of such technique, as image forming apparatuses such as copying machines, printers, and the like attain higher image qualities, a technique for embedding additional information into an image to specify the output device model and model number from the image output onto a recording sheet for the purpose of preventing securities such as banknotes, revenue stamps, and the like from being forged is known.
For example, a technique for multiplexing information by embedding additional information in the high-frequency range of color difference and saturation components with low visual sensitivity has been proposed.
However, the conventional multiplexing technique suffers the following problems.
FIG. 16 illustrates the conventional additional information embedding method. As shown in FIG. 16, image information A and additional information B are multiplexed via an adder 1601 to generate multiplexed information C. The adder 1601 may add information in the real space of the image information A or may transform the image information A into that in the frequency domain using, e.g., Fourier transformation, or the like and may then synthesize the additional information B in the high-frequency range of the converted information.
If it is possible to distribute the multiplexed information C generated in this way without undergoing any image processes such as various filtering processes and the like or any encoding processes such as irreversible compression or the like, it is easy even for the conventional technique to decode the additional information B from the multiplexed information C. Also, image information, which is distributed on, e.g., the Internet, can also be decoded via digital filters for improving image quality such as edge emphasis, smoothing, and the like, as long as it has some noise resilience.
Assume that an image forming apparatus has only expression performance as low as two to several gray levels per color. In recent years, ink-jet printers can express several gray levels per color using inks with lower dye densities or by variably controlling the dot sizes to be output, but cannot express a photo-quality image unless a pseudo halftone process is used.
FIG. 17 illustrates an example that executes a pseudo halftone process upon conventional additional information multiplexing. That is, in addition to the arrangement shown in FIG. 16, the multiplexed information is converted into quantized information D by a pseudo halftone process 1701, and the converted information is printed out onto a recording sheet by a printer output 1702, thus obtaining printed information E that has deteriorated considerably.
Hence, decoding additional information from information on a recording sheet for the purpose of preventing forgery amounts to decoding additional information from the printed information E after the series of processes shown in FIG. 17. However, the image information changes to a large extent by the two processes, i.e., the pseudo halftone process 1701 and printer output 1702. Therefore, it is very difficult to multiplex additional information so as not to be visually detectable, and to normally decode the multiplexed additional information from information on a recording sheet.
In the aforementioned multiplexing technique, information is appended in the high-frequency range of an image. However, when the subsequent pseudo halftone process uses error diffusion, the frequency range of the additional information is buried under that of a texture produced by error diffusion due to the characteristics of a high-pass filter unique to error diffusion, and it becomes harder to decode the additional information. Also, in order to accurately decode the information, a scanner device with very high precision is required.
That is, when the pseudo halftone process must be done, the multiplexing method shown in FIG. 17 is not suitable. In other words, an additional information multiplexing method that fully utilizes the characteristics of the pseudo halftone process is needed.
Some examples that combine multiplexing of additional information and redundancy of the pseudo halftone process will be explained below.
As a first example, upon binarization using ordered dithering, additional data is mixed into an image signal by selecting one of dither matrices indicating an identical gray level. However, in ordered dithering, it is difficult to output a photo-quality image unless a printer having a high resolution and very high mechanical precision is used. Small mechanical precision errors are produced as low-frequency noise such as horizontal stripes or the like, and are easily visually detectable on the paper. When the dither matrices are periodically changed, a specific frequency range generated by a regular dither pattern is disturbed, thus adversely influencing image quality. Also, the decoder must decode by estimating the dither matrix used in binarization while pixel values of image information as an original signal are unknown and, hence, it is hard to attain accurate decoding.
As a second example, a method of multiplexing additional information using a color dither pattern method is known. In this method, deterioration of image quality upon switching the dither matrices cannot be avoided as in the above example. Compared to the first example, a larger number of pieces of additional information can be multiplexed, but color tincture changes since the layout of color components changes, and image quality deteriorates considerably in a solid portion. Also, decoding becomes more harder to attain.
In any case, these methods that change the dither matrices suffer the problem that decoding is hard to attain despite the fact that image quality deteriorates considerably.
The present invention has been made to solve the above-mentioned problems and has as its object to provide an image processing apparatus and method, which can multiplex additional information onto image information while suppressing deterioration of image quality, and can easily decode the additional information, and a storage medium.
In order to achieve the above object, according to a preferred embodiment of the present invention, there is provided an image processing apparatus for multiplexing additional information onto image information, comprising quantization means for quantizing a pixel of interest of the image information by a pseudo halftone process based on error diffusion, and control means for controlling a quantization value of the pixel of interest with quantization values of a plurality of pixels including the pixel of interest forming a specific pattern indicating the additional information.
For example, the specific pattern is a pattern which is not easily generated by the quantization means.
It is another object of the present invention to provide an image processing apparatus and method having a novel function, and a storage medium.